In today's radio communication systems, different measurement techniques are used for estimating channel quality at a radio receiver.
These measurement techniques are employed e.g. in systems using access techniques such as Universal Mobile Telecommunications System (UMTS), and Worldwide Interoperability for Microwave Access (WiMax). WiMAX refers to broadband wireless networks that are based on the IEEE 802.16 standard. One such measurement technique is the Channel quality indicator (CQI). CQI is the term of the downlink quality report and is measured by the user equipment and sent to base station. CQI is an input to several algorithms.
The main purpose of CQI is for transport format selection, i.e. power, block size, modulation and coding. The High Speed Physical Downlink Shared Channel (HS-PDSCH) used in Universal Terrestrial Radio Access (UTRA) transmission power is selected based on the CQI to be received and detected by the user equipment with a certain probability such as typically 10% Block Error Rate (BLER). BLER is a ratio of the number of erroneous blocks to the total number of blocks received on a digital circuit. In case of high load and power limitation a smaller transport block size (TBS) than desired is selected. A smaller TBS means more coding and/or lower order modulation so that the block can be received by the user equipment when transmitted with lower power.
CQI is also used for power control of the High Speed Shared Control Channel (HS-SCCH) channel. Also Media Access Control (MAC) scheduling of packets/users on the shared High Speed Physical Downlink Shared Channel (HS-PDSCH) channel can be based on CQI. Best Signal-to-Interference Ratio (SIR) and Proportional Fair (PF) scheduling are examples of CQI-based scheduling algorithms.
Evolved UTRA (E-UTRA) may also include a similar downlink quality measurement by the user equipment sent to base station and used for Radio Resource Management (RRM) algorithms. There may also be one CQI-measurement per group of sub-carriers.
One major drawback of a transport format selection technique is that it is based on a closed loop measured quality which will be outdated due to processing and transport delays when the block is sent, i.e., the reported CQI at time t, is used to select the TBS of the transport block (TB) sent at time t+d. It is also costly (radio resource and power/battery . . . ) for the uplink to report the CQI each TTI. Typically the CQI reporting interval is less frequent which results in more delays of the measurement.
Also E-UTRA CQI measurements will be delayed from user equipment to base station. More CQI measurements are expected per user equipment for sub-carrier selection and power control purposes. Decreasing the reporting interval will be a costly way to shorten the delay.
WO 2004/042982 discloses a method of improved performance through channel quality prediction for communications systems employing link adaption techniques. The system includes a receiver which makes selective measurements on downlink transmissions, and then stores one or more of the measurements or a channel quality indicator derived there from. The receiver then retrieves one or more of the past measurements (or the past channel quality estimates themselves), and combines it with current measurements (or the current channel quality estimate), to predict what the channel quality will be at some future time and derive a predictive CQI. This predictive CQI, derived from both current channel measurements and at least one past channel measurement, is then sent to the transmitter for use in updating transmission parameters. The problem with this system is that all predictions are done in the user equipment. Doing the predictions in the user equipment is risky since different user equipments could do this in different ways, making the behavior different between user equipments. This could be caused by different prediction techniques used in different user equipments. Changing the reported CQI in the user equipment leaves the base station unaware of what is being reported as CQI to the base station.
US 2005/0227697 depicts techniques and systems for channel prediction in wireless networks. In this system a base station receives a succession of channel indicators from each of a plurality of mobile units, each channel indicator being received from each mobile unit once per timeslot. A channel predictor uses the channel indicators to generate a channel condition prediction for each mobile unit, the channel condition prediction being based on a balanced estimate using the most recent channel condition indicator and the mean of the succession of channel indicators. A weight is computed based on a gradient of the succession channel indicator values and used to assign relative emphasis to the most recent channel condition indicator and the mean channel condition indicator in order to give greater emphasis to the most recent indicator during slowly changing conditions and greater emphasis to the mean indicator during rapidly changing conditions. This prediction technique may work satisfactory in some cases but it is not clear that it will be efficient in typical cases. For example, in slowly varying channel conditions, an extreme channel condition will typically be followed by channel conditions far from the mean. Prediction according to solution may result in too coarse predictions.